New research casts doubt on the commonly held belief that taking more exercise always helps you lose weight.
A study in the journal Current Biology suggests that if you jog for longer or work up a bigger sweat at the gym, your body will adapt to the higher activity level and prevent you shedding any more calories.
Two Different Tools for Weight Loss
The researchers say their findings should act as a reminder that exercise should go hand in hand with diet for anyone trying to lose weight or prevent weight gain.
"There is tons of evidence that exercise is important for keeping our bodies and minds healthy, and this work does nothing to change that message," says Professor Herman Pontzer from City University of New York who led the study. "What our work adds is that we also need to focus on diet, particularly when it comes to managing our weight and preventing or reversing unhealthy weight gain."
The researchers measured the daily energy expenditure of 332 adults over the course of a week. The participants came from 5 different countries across Africa and North America: the US, Ghana, Jamaica, the Seychelles and South Africa.
Energy Expenditure 'Levels Out'
The results showed that exercising more led to a small, but measurable increase in energy expenditure. This link was strongest in those taking low or moderate amounts of exercise. However, for those taking part in the most strenuous activities, energy expenditure plateaued.
People with moderate activity levels were burning about 200 calories more than the most sedentary people, they say, but those who worked out the most saw no reward for their extra work in terms of energy expenditure.
"The most physically active people expended the same amount of calories each day as people who were only moderately active," explains Professor Pontzer.
He says the results don't come as a complete surprise after his experiences working among the Hadza, a population of traditional hunter-gatherers in northern Tanzania.
"The Hadza are incredibly active, walking long distances each day and doing a lot of hard physical work as part of their everyday life," he says. "Despite these high activity levels, we found that they had similar daily energy expenditures to people living more sedentary, modernised lifestyles in the United States and Europe.
"That was a real surprise, and it got me thinking about the link between activity and energy expenditure."
The researchers now plan to study how the human body can adapt to greater physical demands without needing extra calories.
The Role of Exercise
Although the study didn't focus specifically on the role that exercise can take in weight loss, Professor Pontzer believes exercise ''can be part of a successful weight loss strategy". He says: "We need to think about exercise and diet as two different tools. Exercise is good at lots of things, such as maintaining heart health. Diet is going to be the better tool for managing your weight."
His co-researcher, Professor Edward Melanson, from the University of Colorado Anschutz Medical Campus in Aurora, says ''the public health message is not going to change one bit" because working out can help prevent diabetes, keep blood pressure under control and reduce stress. "Exercise does matter [for overall health], whether you lose weight or not," he says.
'Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans', H Pontzer et al, Current Biology.
'Exercise: Is more always better', WebMD Health News, 28th Jan 2016.
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Cite this: Tough Workouts 'May Not Lead to Weight Loss' - Medscape - Jan 29, 2016.